1. Field of the Invention
This invention relates to a depth control device for use with an underwater cable, such as one used in seismic testing.
2. Description of the Related Art
In marine seismic exploration, a cable, commonly referred to as a streamer cable, is towed underwater by a surface vessel. An array of hydrophones is mounted in or attached to the cable, and an acoustic source or gun is fired to force an impulse of compressed air into the water, creating a bubble. The collapse of the bubble generates acoustic pulses that radiate through the water and into the earth. Reflections of the pulses off geologic structures beneath the sea floor are picked up by the hydrophones, and data representing the detected reflections are transmitted to the surface vessel. By analyzing the reflections, it is possible to discover subsea formations of oil or gas.
It is desirable to accurately control the, depth of the cable as it is being towed through the water. For this purpose, depth control devices, commonly referred to as "cable leveling birds" because they bear some visual resemblance to water fowl, are attached to the cable at intervals along its length. The depth control devices are equipped with adjustable diving planes, generally referred to as wings, each having a pitch axis about which it can be pivoted by motors in the depth control devices to adjust the lift generated by the wings so as to maintain the cable at a desired depth.
In a typical depth control device, the force for rotating the wings about their pitch axes is provided by an electric motor coupled to the wings in various manners. For example, the motor may have an output shaft which rotates a lead screw, which in turn pivots a lever drivingly connected to the wings. With such an arrangement, the axis of the output shaft of the motor ends up being located in a different plane from the pitch axes of the wings, e.g., the wings may have to be mounted either above or below a housing containing the motor. This arrangement is not ideal because disposing the motor output shaft and the pitch axes in different planes increases the height of the depth control device, resulting in increased drag, vibrations, and hydrodynamic noise. The increased drag increases the power required to tow the underwater cable through the water, while the increased vibrations and hydrodynamic noise can introduce errors into the acoustic measurements being made by the underwater cable.